arXiv subjects

Xi Chen

Publications and source records attributed to Xi Chen.

At least 1,279 records · Page 71Linked to original sources

arXiv2013

The Complexity of Optimal Multidimensional Pricing

We resolve the complexity of revenue-optimal deterministic auctions in the unit-demand single-buyer Bayesian setting, i.e., the optimal item pricing problem, when the buyer's values for the items are independent. We show that the problem of computing a revenue-optimal pricing can be solved in polynomial time for distributions of support size 2, and its decision version is NP-complete for distributions of support size 3. We also show that the problem remains NP-complete for the case of identical distributions.

cs.GT↗

arXiv2013

Single molecule capturer by doped monatomic carbon chain

The B-doped monatomic carbon chain has fine molecular capture ability to H2O and especially to NO2, better than other doped monatomic carbon chains.At 300 K and 1 atm, the capture probability of the B-doped monatomic carbon chain is appreciable even in a NO2 concentration of 1 p.p.m., and the adsorbates' influence on the quantum transport is notable for the detection. In contrast, the pure monatomic carbon chain shows its invulnerability to N2, O2, H2O, NO2, CO and CO2, and it is incapable for molecule capturing due to too low adsorption ability and weak response on the quantum conductance. In the investigation of these issues, a statistic mechanical model [EPL 94, 40002 (2011); Chin. Phys. Lett. 29, 2012 (080501)] was extended to predict the adsorption and desorption rates of molecules on nanodevices. The theoretical foundation of this model was further discussed and its accuracy was verified by molecular dynamics simulations.

cond-mat.mes-hall↗

arXiv2013

Predicting the chemical stability of monatomic chains

A simple model for evaluating the thermal atomic transfer rates in nanosystems [EPL 94, 40002 (2011)] was developed to predict the chemical reaction rates of nanosystems with small gas molecules. The accuracy of the model was verified by MD simulations for molecular adsorption and desorption on a monatomic chain. By the prediction, a monatomic carbon chain should survive for 120 years in the ambient of 1 atm O2 at room temperature, and it is very invulnerable to N2, H2O, NO2, CO and CO2, while a monatomic gold chain quickly ruptures in vacuum. It is worth noting that since the model can be easily applied via common ab initio calculations, it could be widely used in the prediction of chemical stability of nanosystems.

cond-mat.mes-hall↗

arXiv2013

An age-of-allele test of neutrality for transposable element insertions

How natural selection acts to limit the proliferation of transposable elements (TEs) in genomes has been of interest to evolutionary biologists for many years. To describe TE dynamics in populations, many previous studies have used models of transposition-selection equilibrium that rely on the assumption of a constant rate of transposition. However, since TE invasions are known to happen in bursts through time, this assumption may not be reasonable in natural populations. Here we propose a test of neutrality for TE insertions that does not rely on the assumption of a constant transposition rate. We consider the case of TE insertions that have been ascertained from a single haploid reference genome sequence and have subsequently had their allele frequency estimated in a population sample. By conditioning on the age of an individual TE insertion (using information contained in the number of substitutions that have occurred within the TE sequence since insertion), we determine the probability distribution for the insertion allele frequency in a population sample under neutrality. Taking models of varying population size into account, we then evaluate predictions of our model against allele frequency data from 190 retrotransposon insertions sampled from North American and African populations of Drosophila melanogaster. Using this non-equilibrium model, we are able to explain about 80% of the variance in TE insertion allele frequencies based on age alone. Controlling both for nonequilibrium dynamics of transposition and host demography, we provide evidence for negative selection acting against most TEs as well as for positive selection acting on a small subset of TEs. Our work establishes a new framework for the analysis of the evolutionary forces governing large insertion mutations like TEs, gene duplications or other copy number variants.

q-bio.PE↗

arXiv2013

Fully gapped topological surface states in Bi$_2$Se$_3$ films induced by a d-wave high-temperature superconductor

Topological insulators are a new class of materials, that exhibit robust gapless surface states protected by time-reversal symmetry. The interplay between such symmetry-protected topological surface states and symmetry-broken states (e.g. superconductivity) provides a platform for exploring novel quantum phenomena and new functionalities, such as 1D chiral or helical gapless Majorana fermions, and Majorana zero modes which may find application in fault-tolerant quantum computation. Inducing superconductivity on topological surface states is a prerequisite for their experimental realization. Here by growing high quality topological insulator Bi$_2$Se$_3$ films on a d-wave superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ using molecular beam epitaxy, we are able to induce high temperature superconductivity on the surface states of Bi$_2$Se$_3$ films with a large pairing gap up to 15 meV. Interestingly, distinct from the d-wave pairing of Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$, the proximity-induced gap on the surface states is nearly isotropic and consistent with predominant s-wave pairing as revealed by angle-resolved photoemission spectroscopy. Our work could provide a critical step toward the realization of the long sought-after Majorana zero modes.

cond-mat.supr-con↗

arXiv2013

Evaluating computational models of explanation using human judgments

We evaluate four computational models of explanation in Bayesian networks by comparing model predictions to human judgments. In two experiments, we present human participants with causal structures for which the models make divergent predictions and either solicit the best explanation for an observed event (Experiment 1) or have participants rate provided explanations for an observed event (Experiment 2). Across two versions of two causal structures and across both experiments, we find that the Causal Explanation Tree and Most Relevant Explanation models provide better fits to human data than either Most Probable Explanation or Explanation Tree models. We identify strengths and shortcomings of these models and what they can reveal about human explanation. We conclude by suggesting the value of pursuing computational and psychological investigations of explanation in parallel.

cs.AI↗

arXiv2013

Fast spin control in a two-electron double quantum dot by dynamical invariants

Inverse engineering of electric fields has been recently proposed to achieve fast and robust spin control in a single-electron quantum dot with spin-orbit coupling. In this paper we design, by inverse engineering based on Lewis-Riesenfeld invariants, time-dependent electric fields to realize fast transitions in the selected singlet-triplet subspace of a two-electron double quantum dot. We apply two-mode driving schemes, directly employing the Lewis-Riesenfeld phases, to minimize the electric field necessary to design flexible protocols and perform spin manipulation on the chosen timescale.

quant-ph↗

arXiv2013

Generalized Fibonacci polynomials and Fibonomial coefficients

The focus of this paper is the study of generalized Fibonacci polynomials and Fibonomial coefficients. The former are polynomials {n} in variables s and t given by {0} = 0, {1} = 1, and {n} = s{n-1}+t{n-2} for n ge 2. The latter are defined by {n choose k} = {n}!/({k}!{n-k}!) where {n}! = {1}{2}...{n}. These quotients are also polynomials in s and t, and specializations give the ordinary binomial coefficients, the Fibonomial coefficients, and the q-binomial coefficients. We present some of their fundamental properties, including a more general recursion for {n}, an analogue of the binomial theorem, a new proof of the Euler-Cassini identity in this setting with applications to estimation of tails of series, and valuations when s and t take on integral values. We also study a corresponding analogue of the Catalan numbers. Conjectures and open problems are scattered throughout the paper.

math.CO↗

arXiv2013

Tuning the Conductance of Monatomic Carbon Chain

Ab initio calculations show that the conductance of short monatomic carbon chain can be dramatically modified by adhering a single H, N, or O atom to the chain. For example, the conductance of the pristine chain gets about two orders of magnitude smaller if an H atom is adhered to the chain. By a statistical model, the structure of the carbon chain with the single atom adhered is found to be quite stable at room temperature, indicating that the method can be used to tune the conductance of monatomic carbon chain.

cond-mat.mes-hall↗

arXiv2013

A scheme for realizing continuously tunable spectrum in the visible light region based on monatomic carbon chains

We propose a scheme for realizing the continuously tunable spectrum based on monatomic carbon chains. By hybrid density functional calculations, we first show that the direct band gap of monatomic carbon chains change continuously from 1.58 to 3.8 eV as strain is applied from -5 to 10% to the chain, with separated Van Hove singularity peaks enhanced. To realize this tunability, a realistic stretching device is proposed by contacting the chain with graphene sheets, which can apply up to 9% elongation to the chain, yielding tunable light-emitting wavelengths from 345 to 561 nm.

cond-mat.mes-hall↗

arXiv2013

The fractal nature of the Fibonomial triangle

It is well known that Pascal's triangle exhibits fractal behavior when reduced modulo a prime. We show that the triangle of Fibonomial coefficients has a similar nature modulo two. Specifically, for any $m \ge 0$, the subtriangle consisting of the first $3 \cdot 2^m$ rows is duplicated on the left and right sides of the next $3 \cdot 2^m$ rows, with an inverted triangle of zeros in between. We give three proofs of this fact. The first uses a combinatorial interpretation of the Fibonomials due to Sagan and Savage. The second employs an analogue of Lucas' congruence for the parity of binomial coefficients. The final one is inductive. We also use induction to show that the Fibonomial triangle has a similar structure modulo three. We end with some open questions.

math.CO↗

arXiv2013

Fast and robust population transfer in two-level quantum systems with dephasing noise and/or systematic frequency errors

We design, by invariant-based inverse engineering, driving fields that invert the population of a two-level atom in a given time, robustly with respect to dephasing noise and/or systematic frequency shifts. Without imposing constraints, optimal protocols are insensitive to the perturbations but need an infinite energy. For a constrained value of the Rabi frequency, a flat $π$ pulse is the least sensitive protocol to phase noise but not to systematic frequency shifts, for which we describe and optimize a family of protocols.

quant-ph↗

arXiv2013

Mass acquisition of Dirac fermions in Cr-doped topological insulator Sb2Te3 films

We introduce time-reversal-symmetry breaking by doping Cr atoms into the topmost quintuple layer or into the bulk of Sb2Te3 thin films. We find that even at a high Cr-doping level the Landau level spectrum keeps a good quality, enabling the first demonstration of deviation of the zero-mode Landau level, induced by the acquisition of a mass term in the surface states in the presence of surface or bulk magnetic doping. The magnitude of the mass term in the surface states increases with increasing Cr-doping level. Our observation suggests Cr-doped Sb2Te3 is a promising candidate for the realization of the proposed novel magnetoelectric effects.

cond-mat.mtrl-sci↗

arXiv2013

The smooth center of the cohomology of a singular variety

We study constraints on the Chern classes of a vector bundle on a singular variety. We use this constraint to study a variety which carries a Hodge cycle that are not a linear combination of Chern classes of vector bundles on it.

math.AG↗

arXiv2013

Dynamics of Special Points on Intermediate Jacobians

We prove some general density statements about the subgroup of invertible points on intermediate jacobians; namely those points in the Abel-Jacobi image of nullhomologous algebraic cycles on projective algebraic manifolds.

math.AG↗

arXiv2013

Electronic analogy of Goos-Hänchen effect: a review

The analogies between optical and electronic Goos-Hänchen effects are established based on electron wave optics in semiconductor or graphene-based nanostructures. In this paper, we give a brief overview of the progress achieved so far in the field of electronic Goos-Hänchen shifts, and show the relevant optical analogies. In particular, we present several theoretical results on the giant positive and negative Goos-Hänchen shifts in various semiconductor or graphen-based nanostructures, their controllability, and potential applications in electronic devices, e.g. spin (or valley) beam splitters.

physics.optics↗

arXiv2012

Shortcuts to adiabaticity by superadiabatic iterations

Different techniques to speed up quantum adiabatic processes are currently being explored for applications in atomic, molecular and optical physics, such as transport, cooling and expansions, wavepacket splitting, or internal state control. Here we examine the capabilities of superadiabatic iterations to produce a sequence of shortcuts to adiabaticity. The general formalism is worked out as well as examples for population inversion in a two-level system.

quant-ph↗

arXiv2012

Crystal structure and epitaxy of Bi2Te3 films grown on Si

We report comprehensive x-ray diffraction studies of the crystal structure and epitaxy of thin films of the topological insulator Bi2Te3 grown on Si (1 1 1). The films are single crystals of high crystalline quality, which strongly depends on that of their substrates, with in-plane epitaxial relationships of Bi2Te3 [2 1 -3 0] || Si [1 -1 0] and Bi2Te3 [0 1 -1 0] || Si [1 1 -2] along which the lattices of 1x3 Bi2Te3 and 2x2 Si supercells are well matched. As the samples age, we observe loss of crystalline Bi2Te3 film thickness accompanied with roughening of the crystalline interfaces, formation of new crystalline phases as well as compositional and structural modification of the Si substrate, consistent with the diffusion of Te into the Si substrate.

cond-mat.mtrl-sci↗